The present invention relates generally to gas turbine engines, and, more specifically, to component cooling therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel for generating hot combustion gases in a combustor. Energy is extracted from the combustion gases in a high pressure turbine for powering the compressor, and additional energy is extracted in a low pressure turbine which powers a fan in a turbofan aircraft engine application, or drives an output shaft for marine and industrial applications.
Engine efficiency may be maximized by maximizing the temperature of the combustion gases from which energy is extracted. However, the combustion gases must be contained in the engine by various components which are therefore subject to heating therefrom.
Typical components exposed to the hot combustion gases include the liners of the combustor, the vanes and bands of turbine nozzles, and rotor blades and their surrounding turbine shrouds, for example. These hot components are typically made of state-of-the-art high strength superalloy materials, typically nickel or cobalt based for gas turbine engine applications. These superalloys are expensive, but maximize the high temperature strength of the hot components for achieving the desired long useful life thereof for reducing maintenance operations and corresponding costs.
In conjunction with the superalloy composition of these hot engine components, cooling air bled from the compressor is also used for providing cooling during operation. Various configurations of cooling apertures and channels are provided in these hot components for suitably channeling the pressurized air coolant therethrough for providing internal cooling. The spent cooling air is typically discharged from film cooling holes extending through the inboard or exposed surfaces of the components directly facing the hot combustion gases for providing a thermally insulating cooling air film layer between the component and the hot combustion gases.
These hot components may also be further protected by providing thereon thermal barrier coatings (TBC) which are typically ceramic materials providing additional thermal insulation between the metal substrates of the components and the hot combustion gases.
Thermal barrier coatings are typically applied to the metallic substrates atop a metallic bond coat therebetween, although thermal barrier coatings without bond coats are being developed. The bond coat provides a bonding interface layer for improving the bond of the ceramic thermal barrier coating atop the substrate, and additionally provides oxidation resistance.
The proper operation of the thermal barrier coating requires heat conduction through the coating, through the bond coat, and through the metallic substrate into the cooling circuits which extract heat therefrom. Not only does the metallic substrate have maximum temperature operating limits, but the bond coat and thermal barrier coating also have their respective maximum temperature limits which should not be exceeded for ensuring the desired useful life thereof.
However, the performance of superalloy metallic substrates, and the various forms of conventional thermal barrier coatings and their corresponding bond coats is nevertheless limited by the ability of the air coolant to cool these materials for maintaining them below their maximum operating temperatures. Although the spent cooling air is additionally used in the cooling film for thermally insulating and protecting the thermal barrier coating itself, the thermal barrier coating necessarily requires cooling itself which occurs through conduction to the underlying bond coat and metallic substrate.
Accordingly, it is desired to provide improved cooling of the thermal barrier coating itself when applied atop the metallic substrate.